PROJECT SUMMARY CDX2 is a Drosophila caudal-related homeobox transcription factor that is important for the establishment and maintenance of intestinal epithelial cells. Aberrant CDX2 expression has been linked to gastric intestinal metaplasia, leukemia, and various solid tumors, including up to 90% of colonic adenomas and adenocarcinomas. In the colon, CDX2 has been associated with both tumor suppressor and oncogenic properties. CDX2 likely functions in a context-dependent manner, yet how context affects CDX2 function remains poorly understood. The insulin-like growth factor (IGF) signaling pathway plays a central role in cellular proliferation and survival. IGF activity is inhibited by the insulin-like growth factor binding protein 3 (IGFBP3), a well- described transcriptional target of the tumor suppressor p53. Whereas wild-type p53 transcriptionally activates IGFBP3 expression; mutant p53 does not. Our preliminary data show that in a subset of colon cancers that harbor wild-type p53, CDX2 transcriptionally represses IGFBP3. This leads to the promotion of anchorage-independent colony formation, and enables cellular response to IGF. In contrast, in a subset of colon cancers that harbor mutant p53, CDX2 induces IGFBP3 and apoptosis, and blunts cellular response to IGF. The goal of this proposal is to elucidate the mechanisms whereby CDX2 regulates IGFBP3 in different p53 contexts; and the ensuing consequences on the IGF axis and growth. We hypothesize that in colon cancers with WT-p53, CDX2 transcriptionally represses IGFBP3, promotes anchorage-independent colony formation, and enables IGF signaling. Conversely, in colon cancers with MUT-p53, CDX2 has the opposite effect. Three specific aims are proposed to test the above hypotheses: Specific Aim 1: To determine the mechanisms by which CDX2 and p53 regulate IGFBP3. Specific Aim 2: To determine the effects of CDX2 and p53 on the IGF axis. Specific Aim 3: To determine the effects of CDX2 and p53 on tumor growth and response to IGF axis inhibitors. 1